1. Field of the Invention
This invention relates to metal casting of alloys using the investment casting process. In particular it relates to the rapid solidification of aluminum alloy castings.
2. Description of Related Art
Investment or “lost wax” castings are increasingly specified for demanding applications favouring a combination of tight tolerances, good surface finish, thin walls and high integrity. Integrity is characterized by an absence of voids or macro defects, and possessing predictable and elevated levels of mechanical properties. In order to achieve high integrity castings capable of meeting the uppermost level of mechanical properties on a statistical basis, special techniques are required to solidify the casting inside the precision ceramic mold. Considerable prior art describes controlled solidification of castings in order to improve final part performance, none of which are ideal for the investment casting process. The invention covers a unique casting solidification method.
Mechanical properties (strength, ductility, fatigue resistance, etc.) of cast alloys are often inferior over similar wrought alloys, due to an associated coarse microstructure (large grainsize or dendrite arm spacing) of the casting, resulting from slow solidification. The relatively insulating investment casting shell mold combined with superheated alloy being poured into the pre-heated mold results in only moderate cooling rates. Accelerating the speed of casting solidification will improve mechanical properties of many alloys dramatically. Cast aluminum-silicon-magnesium alloys for example exhibit superior static and dynamic mechanical properties, and an associated reduction in data scatter, when solidified in a rapid controlled manner. Solidification front advancement in a controlled manner is essential in avoiding shrinkage defects which would otherwise lower mechanical properties of the component.
Extraction of superheat from a solidifying investment casting necessitates the use of chills adjacent to the casting surface, or use of a cooling medium acting on the exterior surface of the thin ceramic shell mold. Many processes have been devised to cool the exterior of the investment casting shell mold, following filling of the mold with hot metal alloy.
U.S. Pat. No. 6,308,767 (December 1999) by Hugo, Betz and Mayer describes a process whereby an investment casting is directionally solidified in a liquid metal bath inside a vessel. European Patent 0,571,703, B1 (November 1996) by Folkers, Nicolai, Rodehuser, Steinrucken, and Henneke describes a process whereby a cast mold is lowered into a bath of water/organic liquid mixture inside a vessel. U.S. Pat. No. 4,108,236 (August 1978) by Salkeld describes the use of a floating baffle to separate the cast ceramic shell from the liquid metal bath quenchant for directional solidification of the casting. U.S. Pat. No. 3,915,761 (October 1975) by Tschinkel, Giamei and Kear describes a process of lowering a cast mold into a cooling bath in order to achieve directional solidification of the casting. U.S. Pat. No. 6,622,744 (September 2003) describes a process of lowering a cast mold into a bath of cooling oil to extract heat from the mold.
Prior art techniques involve for example use of liquid metal heat transfer media for cooling of the shell mold, which in the case of light alloy castings such as aluminum would result in a net inward crushing pressure on the ceramic shell mold and likely failure of large cast articles immersed to great depths. Although heavier and stronger shell molds could be fashioned to resist this inward crushing pressure of the liquid metal bath, heavier shell thickness would reduce conductive heat transfer from the solidifying casting, and make less effective the described process. Prior art techniques using non-metallic quenches, also employ cumbersome manipulation and vertical movement of the mold during solidification thereby risking breakage of the casting, or disturbance of the solidification front. Prior art techniques also expose the solidifying casting to the ambient air environment which risks hydrogen (humidity) absorption into the liquid metal and subsequent generation of porosity into the casting.